Fluidized bed coating process and apparatus



wat 5M ATTORNEY R. A.-. cRRAN FLUIDIZED BED COATING PROCESS ANDAPPARATUS Filed April 29, 1966 United States Patent O 3,483,011FLUIDIZED BED COATING PROCESS AND APPARATUS Robert A. Curran,Framingham, Mass., assgnor, by

mesne assignments, to Solitron Devices, Incorporated, a corporation ofNew York Filed Apr. 29, 1966, Ser. No. 546,253 Int. Cl. C23c 1/00; H01b1/02; B05c 3/00 U.S. Cl. 117-21 5 Claims ABSTRACT OF THE DISCLOSURE Amethod for producing a uniform coating of a heat fusible material uponthe surface of a panel comprising heating said panel unevenly to producea temperature gradient from one edge to the opposite edge, and whilesaid temperature gradient exists, immersing said heated panel into afluidized bed of particles of a heat fusible material and withdrawingthe coated panel.

This invention relates to the production of coatings and particularly tomethods of and apparatus for producing coatings 'by iluidized bedtechniques.

In the uidized bed coating processes, an article to be coated ispreheated at least to the fusion or melting temperature of a fusiblecoating material and then imrn'ersed in a iluidized bed of the coatingmaterial. The iluidized bed is formed by passing an ascending stream ofgas through a body of solid pulverulent coating material. Coatings ofdielectric materials, particularly organic plastics, formed by fiuidizedbed techniques find utility as supporting substrates for electricalcircuitry, particularly, printed circuits. Coatings of boththermoplastic and thermosetting resins may be applied to a relativelylarge metal panel, that may include openings, to form a support for apattern of electrical conducting elements comprising a printed circuitand/ or electrical components mounted on the panel. The thickness anduniformity of the dielectric coating on the panel may be critical,particularly in the case of printed wiring boards with componentmounting holes with close diameter tolerances and sections whichfunction as male plugs and have close thickness tolerances.

.An object of the present invention is to provide a novel and improvedmethod of producing coatings having accurately controlled and uniformthickness on panels by uidized bed coating techniques.

In the application of coatings to flat panels by fluidized bedprocesses, the heated panel is usually suspended in a generally verticalplane while being immersed in the iluidized bed so that the lowerportion of the panel is introduced first and withdrawn last, Withrelatively large panels and short immersion times, this means that thelower portion of the panel is immersed in the iluidized bed for asubstantially longer period than the upper part of the panel and willresult in an uneven coating because the fusible particles begin to meltand adhere to the heated panel immediately as they come into contactwith the panel. The larger the panel and hence, the greater theimmersion time differential between portions of the panel, the greaterthe variation in thickness of the coating applied to the panel.

Other objects of the invention are: to provide a method of uidized bedcoating in which the panel to be coated is heated differentially toprovide a temperature gradient from one edge of the panel to theopposite edge cor- Patented Dec. 9, 1969 responding to the variation inimmersion time of the sections of the panel in the lluidized bed; toprovide novel and improved means for heating the panel to establish thedesired temperature gradient; and to provide means for differentiallyheating a metal panel with suiiicient rapidity to preclude normalizationof the temperature throughout the panel.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process involvingy the severalsteps and the relation and order of one or more of such steps withrespect to each of the others which are exemplified in the followingdetailed disclosure, and the scope of the application of lwhich will beindicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIGURE 1 is a perspective view, partially in section, of heaterapparatus embodying the invention;

FIGURE 2 is a somewhat diagrammatic view of the apparatus of FIGUR-E 1;and

FIGURE 3 is a sectional view through a iiuidized bed tank illustratingthe method of the invention.

The rate of build-up of a coating on a panel immersed in a iluidized bedis dependent upon a member of factors, particularly the temperature andheat content of the preheated panel, while the thickness of the coatingis a function of the rate at which the coating builds up and the lengthof the period of immersion in the iluidized bed. When a panel is moved,lower edge foremost, downwardly into a yiiuidized bed and then removed,upper edge foremost, in the opposite direction, the lower portion of thepanel will experience an immersion period that is longer than theimmersion period for the upper portion of the panel. This discrepancymust exist even though the relative proportions of the two extremes ofimmersion may vary with changes in the speed of introduction andwithdrawal, the dwell time within the iluidized bed and the verticaldimension of the panel. For relatively large panels and short dwelltimes, the difference in immersion periods for upper and lower portionsof the panel may be suiicient to cause very substantial variations inthe coating thickness and the diameters, after coating, of holes in thepanel.

In accordance with the invention, uniform coatings are provided on thesurface of a panel by preheating the panel differentially to create atemperature differential in the panel ranging from a low (at least equalto the fusion temperature of the coating material) at the lower edge ofthe panel to a high at the upper edge of the panel. This temperaturedilferential corresponds proportionally to the differences in immersiontimes for all portions of the panel so that coating bui1dup isrelatively slower on the lower portions of the panel and more rapid onthe upper portions of the panel. In other words, the rate 0f coatingbuild-up is inversely proportional to the immersion time.

Although supporting panels useful as substrates for printed circuitrymay be formed of substantially any material having the requisitephysical properties and capable of withstanding the elevatedtemperatures necessary for iluidized bed coating, metals, particularlyaluminum, are preferred. Of course, metals are relatively goodconductors of heat so that a metal panel must be heated rapidly in orderto produce the requisite temperature gradient and immediately immersedin the fluidized bed while the temperature gradient still exists.

Reference is now'made to the drawings wherein there is illustratedapparatus for performing the method of the invention including a heaterfor differentially heating fiat panels to produce a temperature gradientin the panels. The heating means of the apparatus comprise a pair ofplatens and 12 mounted in juxtaposition with one of the platens,designated 12, being movable toward and away from the other platen forclamping a panel between the platens. Movement of platen 12 with respectto platen 10 is effected by mounting the panel on a pair of pivotal arms14, in turn actuated by suitable means such as a hydraulic piston andcylinder. A typical panel 16 is shown clamped between the platens, isgenerally rectangular in shape and includes an upper section having anopening in which a hanger 18 may be engaged for suspending andtransporting the panel during operations thereon.

Each of platens 10 and 12 is made up of a plurality (five are shown) ofmodular units designated by the letters a through e. Each modular unitextends from side to side of the platen, includes a separate heatingelement 20, and is thermally insulated from adjoining modular units. Theunits are formed of a thermally conductive material, eg., steel,substantially thicker than the panels so as to have a substantiallyhigher heat content than the panels. Heating elements 20 are of theelectrical resistance type and each is embedded within a platen moduletogether with a thermostat 22 for controlling the temperature of theparticular modular unit. insulating layers 24, formed for example, ofpolytetrauoroethylene, between adjacent heating modules. Layers 24 arepreferably quite thin and function to maintain the temperaturedifferential between adjacent modules. The heating modules of one platenare located in alignment with modules of the opposite platen and theadjacent modules of the two platens are maintained at substantially thesame temperature so that the portion of the panel clamped between a pairof modules will be heated to approximately the temperature of those twomodules.

In practicing the method of the invention, a panel 16 is clamped betweenthe two platens for a period which is preferably just sufficient toraise the temperature of each portion of the panel to the temperature ofthe heating modules between which the particular portion of the panel isclamped. The panel is immediately removed from between the heatingplatens' since allowing it to remain there for a longer period wouldresult in heat transfer from the hotter sections of the panel to thecooler sections of the panel and would eventually result in reduction ofthe temperature gradient, possibly to the extent of normalizing thetemperature throughout the entire panel. In the manner described, thetemperature gradient produced is relatively even due to heat conductionwithin the panel and the provision of a relatively large number ofheated platen modules maintained at a temperature differing bycomparatively small amounts.

The preheated panel immediately upon withdrawal from between the platensis immersed as shown in FIG. 3, in a uidized bed of the fusible coatingmaterial. This uidized bed is contained in a tank 26 having a porousdividing wall 28 near its bottom dividing the tank into a lower chamber30 into which a gas under pressure is introduced, and an upper chamber32 containing the finely divided fusible coating material. Porous 4wall28 permits the gas under pressure to ascend through the powdered coatingmaterial causing the latter to behave as a fluid and contact all exposedsurfaces of the panel when the latter is immersed in the uidized bed.Panel 16 is suspended in a generally vertical plane during both thepreheating and immersion operations so that the lower portion of thepanel is introduced into the fluidized bed irst and withdrawn from theuidized bed last, and thus is immersed in the uidized bed for a longerperiod than the upper portion of the panel. In the coating operation thepanel is introduced as rapidly as possible into the fluidized bed, isretained therein during a period of i predetermined duration and thenwithdrawn as rapidly as possible from the uidized bed. The proportionaldifference in immersion periods for the upper and lower portie-ns of thepanel will, of course, depend upon the size of the panel, the rate atwhich the panel is moved during introduction and withdrawal and thedwell time -within the fluidized bed. If the panel were heated to auniform temperature throughout, and this is particularly true ofrelatively large panels, the thickness of the coating applied to thesurfaces of the panel would vary substantially from top to bottom of thepanel and since the coating is also applied to the walls of openings inthe panel, the finished size of openings, originally of the same size,would als-o vary depending upon the location of the openings relative tothe top and bottom of the panel. However, by establishing a temperaturegradient in the panel ranging from a temperature at least equal to thefusion temperature of the coating material at the lower edge of thepanel to a higher temperature near the upper edge of the panel, thebuild-up of coating material on each section of the panel proceeds at arate proportional to the immersion so that coating is of uniformthickness.

As an illustrative example of the method of the invention, a coating ofa polyester resin with a thickness of .008 was produced on a l2 X 8 X.030" aluminum panel. The coating thickness and hole diameters weremaintained within a tolerance of .0005 for all portions of the panelincluding tabs or extensions of the panel. The panel was preheated to aminimum temperature of 480 F. to produce a gradient of 60 F. along the8" dimensions by clamping the panel between a pair of platens comprising2" modules separated by 1/16" thick insulating layers and maintained attemperatures differing by 20 F. The panel was clamped between theplatens for a minimum of 4.8 seconds in order to preheat the panel tothe desired temperature. However, it was found that the panel could beretained between the platens for as long as 7 seconds without adverselyaffecting the temperature gradient, thereby demonstrating suicienttolerance in the preheating time to make mass production feasible. Arange of preheating times for a particular panel and coating material isreadily determinable by simple experimentation. In the example given,the coating (.008H thick) was produced allowing for a transfer time fromplaten heaters to the uidized bed of 2 seconds and the coating materialemployed was a polyester resin in the B stage for which the recommendedpreheating temperature is 440 F.

Since certain changes may be made in the above process and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is: 1. The method of producing a uniform coating of aheat fusible material upon the surface of a panel comprising:

heating said panel unevenly to produce a temperature gradient thereinranging from a low temperature at one edge of said panel at least equalto the fusion temperature of said material to a substantially highertemperature at the opposite edge of said panel;

while said temperature gradient exists, immersing said heated panel withsaid one edge foremost into a fluidized bed of finely divided particlesof said heat fusible material to cause said particles to melt and adhereto the surface of said panel to form a coating thereon;

retaining said panel immersed in said iiuidized bed for a predeterminedperiod; and

thereafter, withdrawing the coated panel with said opposite edgeforemost, from said fluidized bed.

2. The method of claim 1 wherein said panel is suspended vertically withsaid one edge beneath said opposite 5 edge during immersion in andwithdrawal from said uidized bed.

3. The method of claim 1 wherein said panel is formed of metal and isheated and immediately immersed in said iuidized bed with suicientrapidity to insure continuance of said temperature gradient at leastduring introduction of said panel into said iluidized bed.

4. A method as defined in claim 1 wherein said panel is heated bymaintaining sections of juxtaposed platens at different temperaturesapproximately equal to the temperatures to be produced in correspondingportions of said panel and clamping said panel between said platens.

5. A method as deiined in claim 4 wherein the heat content of saidplatens is substantially greater than the heat content of said panel.

References Cited UNITED STATES PATENTS 3,218,184 11/1965 Lemelson117--21 3,282,249 1l/1966 Ramsay 117-21 3,291,631 12/1966 Smith 117-21FOREIGN PATENTS 639,666 4/ 1962 Canada.

661,766 4/1963 Canada.

WILLIAM D. MARTIN, Primary Examiner RAYMOND M. SPI-EER, AssistantExaminer U.S. C1. X.R. 117-113, 230

